1. Field of the Invention
The present invention relates to an improvement in a photo-field effect transistor (hereinafter referred to also as a “photo-FET”) in which a photodiode and a field-effect transistor (hereinafter referred to also as a “FET”) are integrated monolithically.
2. Discussion of the Background
A photodetector in a near-infrared region, particularly a photodetector array in which plural photodetectors are arranged one- or two-dimensionally, has been in high demand as an infrared photodetector for spectroscopic systems or an infrared camera in a variety of applications in the fields of medicine, disaster prevention and industrial inspection. For instance, in the field of medicine and biometrics, a non-invasive behavior has been reaffirmed and commercialized into such as an “In vivo oxygen monitoring device” or “Authentication system for blood vessel figure” utilizing the spectroscopic characterization of Hemoglobin between 0.7˜0.9 μm in the near infrared region. An applicable scope may be expanded to a diagnosis with Optical Topography or a biometrics when a detection wavelength is set to an infrared region of 1.2 to 1.5 μm. Under such conditions, in-vivo permeability is increased further, and a so called “eye safe” condition will be realized even if someone contemplates the light source.
Also, a detection system for weak light in an infrared region has been in great demand in relation to the single molecule detection by a fluorescent label that has recently received a lot of attention, in view of the fact that the in-vivo emission wavelength by singlet oxygen is 1269 nm etc. Also, in the field of disaster prevention and security, a relatively strong lighting is possible. Therefore, a night-vision camera is expected to be realized coping with the recognition of a living body and a temperature measurement function utilizing the specific infrared photo-absorption properties of a substance such as temperature and moisture. Furthermore, when the wavelength range is expanded up to 5 μm, it becomes possible to detect thermal images or poisonous gases, such as CO, and to work for disaster prevention and remote sensing.
Another important field of application is that of distance recognition or moving body recognition using an imaging device having a function called a “smart pixel.” As disclosed in Document 1, for example, a camera has been developed that measures a distance from the phase of modulated light with a frequency of several MHz. This camera performs a so-called lock-in detection using a switch synchronized in a modulation frequency with a silicon CCD (Charge Coupled Device).
Document 1: Robert Lange and Peter Seitz, “Solid-State Time-of-Flight Range Camera”, IEEE JOURNAL of QUANTUM ELECTRONICS, VOL. 37, NO. 3, pp. 390-397 (MARCH 2001).
Here, when looking to the structure of a fundamental photodetector per se, the conventional photodetector capable of detecting the infrared region is classified roughly into types 1) to 3) noted below.
1) A photomultiplier tube (PMT) of a type multiplying electrons emitted from a photoelectric conversion surface by incidence of light to detect electric charge, or a CCD camera with an electron multiplying mechanism (for example, Electron Bombardment CCD Camera: EB-CCD camera produced by Hamamatsu Photonics K.K.).
2) A PIN photodiode that detects a photoexcited current in a compound semiconductor.
3) An avalanche photodiode that multiplies a photoexcited current within a semiconductor.
In the case of the PMT and avalanche photodiode, however, there are intrinsic problems to establish a detector array, since a highly accelerated voltage is required for accelerating and multiplying electrons generated by light in a vacuum or solid substance, and since there are considerable variations in the multiplication characteristics.
Also in the EB-CCD camera, since an induced electrostatic discharge damages a micro CCD at several volts, it is actually difficult to combine the CCD with an electron multiplying plate requiring an accelerated voltage of 1 KV. Therefore, the EB-CCD camera as a product is too expensive to reach a level that satisfies extensive demands. While a PIN photodiode using a compound semiconductor is simple in structure and relatively easy to establish integration, however, detection limit of the PIN photodiode is much inferior to a silicon CCD. Further, the PIN photodiode is low in sensitivity and is greatly affected by a readout noise from an external amplifier, and has no charge storage mechanism, unlike the silicon CCD.
Another serious problem posed by a background photodetector is that a sensitive wavelength region is limited. Though an image pickup device has been developed in various aspects within a visible region, the image pickup device has insufficient sensitivity at an ultraviolet region in the wavelength of 150 nm to 300 nm or at an infrared region in the wavelength longer than 1 μm. The image pickup device has been required to dispose a plurality of photodetectors having different sensitive characteristics to cope with a wavelength region over a wide range, resulting in a complicated optical system.
In a photodetector having a photosensitive layer of silicon and coping with wavelengths from visible to near-infrared regions, but not a compound semiconductor-based photodetector, a phototransistor that amplifies a photo-induced current with a transistor integrated within a semiconductor is used in an optical relay or image pickup device. Particularly, a CMOS image sensor is increasingly popular as an image pickup device for a high-resolution camera or video camera. The CMOS image sensor is suited for high density integration since it adopts an active cell system having a photodetector and a MOSFET combined within a single pixel.
Furthermore, as disclosed in Documents 2 and 3 listed below, there is a silicon-based photodetector called a VMIS (threshold Voltage Modulation Image Sensor), in which it is intended to combine a photodetector and an FET by interconnecting an output from a built-in photodiode with the back gate of a MOSFET using a p-type well inside of the device.
Document 2: JP-A 2004-241487.
Document 3: “Principle of Operation of VMIS,” Transistor Technology, p. 160, February 2003, published by CQ Publishing Co., Ltd. having a business place at 1-14-2 Sugamo, Toshima-ku, Tokyo.
However, the photogenerated carriers are dissipated without any countermeasures because the respective contact parts of the source and drain have to be deservingly deprived of silicon oxide film for a gate insulator. For this reason, it is necessary to adopt an additional process of providing a hole-storage layer around the gate and source electrodes and an impurity concentration gradient in the lateral direction, thereby producing a potential barrier that prevents the carriers from flowing out of the source electrode. To form such impurity distributions, it is additionally required to perform multiple ion implantation processes. The device isolation process of the VMIS is rather difficult since it is a bipolar device. Thus, the device fabrication process becomes complicated as compared to an ordinary CMOS image sensor.
As an integrated image sensor using a compound semiconductor device having sensitivity within an infrared region, an infrared camera using a Focal Plane Array (FPA) having an integration scale from 320×256 pixels to a VGA (640×480 pixels) has been developed. This infrared camera is lightweight and highly sensitive, but exhibits less persistence of vision as compared with a conventional infrared camera of an image pickup tube system, and thus has been gradually growing in popularity. However, since the light-receiving device per se used in an FPA has no addressing function, realization of an image pickup device requires complicated manufacturing operations such as a wafer thinning process of a two-dimensional compound semiconductor PIN photodetector array and bonding it to a two-dimensional silicon-charge amplifier array. Furthermore, the FPA has not yet solved the problem of a large influence of a readout noise by an external amplifier since it is required to amplify a minute current induced by photo-generated electron-hole pairs corresponding at most to the number of photons in the incident light, similarly to the case of a discrete PIN photodetector.
Highly sensitive photodetectors incorporated with the active devices, such as a heterojunction bipolar transistor (HBT) and a High Electron Mobility Transistor (HEMT), have been studied as compound semiconductor devices having the sensitivity within an infrared region in parallel. Among them, an FET has a simple structure and has been widely utilized mainly as a unipolar device for the application of high-frequency and radio transmission. There is a fair probability of providing a high-speed and low-power-consumption device with ease of integration and enlargement in device area. For example, the FET can also be incorporated as a unit device structure including a part of the photodetector. Furthermore, wide spectrum-range sensitivity can be expected because the essential operation region is near the front surface. In fact, a large number of research results can be confirmed as described below.
For example, a photo-FET having a compound semiconductor-based FET as a basic structure has been developed initially in the GaAs/AlGaAs-based material on a GaAs substrate. However, when GaAs layers are used as buffer layers that act as a channel region for a current pass and also for a light incoming region, the photo-sensitive wavelength is limited up to 850 nm, as described in Document 4 noted below.
Document 4: Hongjoo Song, Hoon Kim, “Analysis of AlGaAs/GaAs Heterojunction Photodetector with a Two-Dimensional Channel Modulated by Gate Voltage,” Extended Abstract of the 2003 International Conference on Solid State Device and Materials, Tokyo, 2003, pp. 186-187.
In view of the above, a material containing indium, such as In0.53Ga0.47As, InGaAsP, In0.52Al0.48As, etc., has been put to use in an effort to ensure a further high speed and an enlargement in a sensitive wavelength range. In the case of InGaAs-based material on an InP substrate, the wavelength range is limited to 2.5 μm even at strained condition. However, an FPA using a photosensitive layer of InSb or InAsSb having sensitivity at a wavelength from 4 to 6 μm has been developed and is being used for thermal-imaging or poisonous-gas sensing.
In GaAs/AlGaAs-based materials, a deep impurity level is induced at the interface between a substrate and an epitaxial layer or between a surface after completion of epitaxial growth and a dielectric insulation film. Further, the GaAs/AlGaAs-based material pins the Fermi level in the vicinity of the midgap. Therefore, carriers are depleted and a semi-insulating layer is formed at the interface. In a material containing indium, in contrast, a conductive layer is liable to be formed on the surface or interface.
In other words, it is required to introduce a donor capable of compensating the influence of a surface level induced at a device interface to obtain conductivity of a channel when a GaAs/AlGaAs-based FET is fabricated. Inversely, it is necessary to suppress the leakage current induced by the conductive layer formed at the surface or interface when an InGaAs/InP-based FET is fabricated.
Since a Schottky barrier is easy to form in InAlAs lattice-matched with InP, an FET using an InGaAs channel on an InP substrate is proposed as described in the below Document 5. Barrier layers sandwiching a channel layer are generally formed of InAlAs. A highest-speed HEMT device has been realized at present based on this material.
Document 5: Yoshimi Yamashita, Akira Endoh, Keisuke Shinohara, Kohki Hikosaka, Toshiaki Matsui, Satoshi Hiyamizu, and Takashi Mimura, “Pseudomorphic In0.52Al0.48As/In0.7Ga0.3As HEMTs with an Ultrahigh for 562 GHz, IEEE ELECTRON DEVICE LETTERS, VOL. 23, NO. 10, P. 573 (OCTOBER 2002).
This device, however, has some drawbacks. Since the InGaAs channel layer that serves as a light-absorbing layer is thin, the absorbing efficiency for the long-wavelength light is not good and an improvement such as waveguide coupling is necessary. The waveguide coupling, in which the device facet is employed as an acceptance surface, requires precise alignment. Furthermore, since a barrier layer of InAlAs reacts with oxygen or moisture in the atmosphere, the device poses a problem from the standpoint of reliability. Another problem is that the threshold value is apt to be instable since impurities in the InAlAs are moved easily.
While Al-free InP is advantageous as a gate material from the viewpoint of reliability, some difficulties remain: a Schottky junction is difficult to form; a leakage current is induced from the surface and from the interface between the substrate and the epitaxial layer.
These problems have already been recognized, and some solutions have been proposed. Document 6 noted below, for example, discloses a photo-FET having an HEMT structure as its fundamental structure, in which the pinning effect exerted on a surface potential in the presence of p-type InAlAs is used to elongate a depletion layer from the surface in a dark state. Thereby, the photo-FET having an HEMT cuts off a source-drain current and induces electrons through the accumulation of holes in the gate region at the time of light illumination to thereby make an attempt to fabricate a photodetector having an amplification function.
Document 6: JP-A 2001-111093.
In addition, Document 7 noted below discloses an improvement in an ordinary FET without bringing consciousness to the construction of a photo-FET, in which a trench is formed to reach an InP substrate to eliminate the influence by conduction defects at the interface between the substrate and an epitaxial layer.
Document 7: JP-A HEI 5-275474.
Furthermore, the current path is restricted using a quantum wire as disclosed in Document 8, proposed by the present inventors, and as disclosed in Documents 9 and 10, noted below. A photo-FET is restricted using a quantum point contact to enhance light detection sensitivity significantly as compared with the background art.
Document 8: JP-A 2005-203428.
Document 9: JP-A 2001-332758.
Document 10: JP-A HEI 9-260711.
Concerned about a photodetector based on a compound semiconductor-based FET structure, excluding a silicon-based one, it is still impossible to obtain a photo-FET having a sufficiently satisfactory optical sensitivity even when the technique disclosed in Document 6 is used along its gist to fabricate a device, which is considered as being excellent in the background art documents. In particular, it is necessary to form a sufficiently thick photo-absorption layer to materialize a front side illuminated photo-FET applicable to an image pickup device. In addition, precaution against a leakage current has not been taken and a leakage current from the edge of the channel width direction orthogonal to the channel length direction has not been suppressed. Even when the trench-digging technique, as disclosed in Document 7, has been incorporated therein, this cannot be effective according to the experiences of the present inventors because the formation of a dielectric insulation film induces leakage of a current through the surface of the trench.
The structures as disclosed in Documents 8, 9, and 10 have to be formed through ultra fine microfabrication technique using the electron beam exposure method. In addition, since a sub-micron photo-lithography machine is necessary to form a current-restricted part as disclosed in Document 10, minute adjustment of a gap spacing is required. It is, as a matter of course, undesirable to require such high accuracy for the device fabrication, because of the large number of steps required and their complexity. Since satisfactory optical sensitivity has not yet been obtained, the background art techniques have not yet reached a level that can sufficiently be advocated in future without any modification. To begin, the quantum structure is not indispensable to the acquirement of a highly sensitive photo-FET.